a. Field of the Invention
Broadly speaking, this invention relates to the measurement of radio frequency interference. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for accurately measuring the radio frequency interference generated by operating electrical or electronic apparatus and equipment.
B. Discussion of the Prior Art
As is well known, virtually all electrical and electronic equipment generates a certain amount of radio frequency interference (RFI) when operating. By careful design and the use of screening, the unwanted RFI can be minimized, but not completely eliminated. It thus becomes important to accurately measure the RFI generated by a particular piece of equipment to determine if it is compatible for use with other electronic equipment which may be sensitive to even low levels of RFI.
The specifications on permissible limits of spurious radiation from electrical and electronic equipment are typically stated in terms of spectral field intensities with reference to free space or half space conditions. However, in practice, the RFI pollution that exists in free space hinders the determination and measurement of spurious RF radiation from potential RFI sources and, thus, handicaps the enforcement of meaningful specifications and regulations on electromagnetic compatibility.
To overcome this problem, the so-called "screen room" was developed to provide both shielding from external RF interference and an environment in which standardized RFI test procedures could be developed for all types of RFI sources.
While the screen room has been highly successful in shielding the test apparatus from external interference it has not, unfortunately, led to the development of practical test procedures, particularly when the dimensions of the screen room and the wavelengths of the interference emitted from the test object become compatible. In other words, within the confinement of the screen room proximity effects, wall interaction, and mutual coupling between the test object and the measuring object tend to yield erroneous measurements.
The problem, then, is to devise a test environment and a testing procedure which are free from both the RFI interference in free space and perturbations caused by the test instruments themselves.